Update block header serialization

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Either some things have changed or there was some discrepancies in the port from go, They are using a pointer for signatures and the nil value gets serialized as null in Cbor, which is equivalent to Option or None in rust. They were checking the nil if bytes are empty, which doesn't translate to Rust or how we had our types setup
  • Also found a bug in the deserialization of Signature that I did a while ago, if bytes are empty it will panic on .remove(0) because that impl expects a valid serialization

Edit: Also updates EPoStProof serialization as discussed, and removed the tests that were statically setup for the old serialization

Edit2: Added test runner for block header serialization vectors, but the vectors were a little broken so I switched the submodule to be my fork for now

Reference issue to close (if applicable)

Closes

Other information and links

[dutterbutter]

Should prob move this under chainsafe

[ec2]

looks good other than dustins comment on putting the vectors under chainsafe